Sediments and sedimentary rocks can be considered as geological archives that faithfully reflect their provenance information if the bias introduced by physical and chemical processes during transport and deposition can be properly recognized and corrected for. The sediment provenance analysis both in modern and ancient settings is crucial to trace the sediment sources, reconstruct paleoclimate and paleoenvironment, and interpret the evolution of the Earth’s surface. Modern sediments, unaffected by diagenesis and eroded, tansported and deposited under climatic conditions that are fully known, can provide valuable information on the interactions among the different controlling factors that govern source-to-sink systems. Rivers draining the Himalayan orogen provide the good opportunity to trace the source fingerprinting that is documented in modern fluvial and eolian sand and how these signatures reflect the erosion patterns of the modern and paleo-river systems. A multidisciplinary approach based on petrography, minerology, geochemistry and geochronology is emphasized in this research, in order to obtain a comprehensive provenance information. Our research area focused on the modern sands from two river system: Yarlung River and Indus River. In the Yarlung River system, the Nian River was chosen to investigate the petrographic, mineralogical and chronological signature of sediments from Tethys Himalaya, Greater Himalaya, Kangmar gneiss dome and Transhimalayan ophiolitic suture. Different tectonic domains are characterized by distinct heavy mineral assemblages, e.g., the first-cycle sillimanite and garnet in Greater Himalaya, and clinopyroxene, olivine and enstatite in the forearc ophiolites. Sand carried by the Nian River and its major tributaries, mainly reflects Tethys Himalayan characteristics, consistent with the geochronological results. Erosion rates were also evaluated and circumscribed in the middle Yarlung River catchment. The average erosion rate in the Nianchu catchment is estimated at 0.07-0.10 mm/a, twice as that in the middle Yarlung and Lhasa River catchments, which is principally ascribed to the high erodibility of Tethys Himalayan strata. In the Indus River system, minerochemical analysis of amphibole, garnet, epidote and pyroxene grains, and geochronological analysis of detrital zircons, associated with analysis on petrography, bulk-sediment geochemistry and isotopic geochemistry, in aolian sand from Thal Desert and fluvial sand in selected tributaries draining one specific tectonic domain in the upper Indus catchment, were carried out to discriminate compositional signatures, decipher the provenance information, and trace the erosional evolution of the western Himalaya syntaxis. The compositional fingerprints of Thal Desert sand are characterized by litho-feldspatho-quartzose to quartzo-feldspatho-lithic detrital modes and very rich amphibole-dominated heavy-mineral assemblages. The high heavy mineral concentration, less negative εNd, abundant zircon ages at 40-100 Ma, and specific mineral varietal fingerprints, consistently reflect that the Kohistan arc has acted as the main sediment source. Karakorum appears to contribute less while Himalaya shows higher influence on the Thal Desert sands than modern river sands from the Indus. As a Quaternary repository of wind-reworked Indus River sand at the entry point in the Himalayan foreland basin, Thal Desert sands document higher erosion rates than today in the glaciated areas formed largely by batholites granitoids of the Asian active margin. The close compositional and chronological connection between the Thal Desert and the ancient Indus Delta and Fan deposits, shed new light on the reconstructing of paleodrainage and the understanding of relationship between climatic and tectonic forcing that controlled the erosional evolution of the western Himalayan-Karakorum orogen.

L’oggetto di questa tesi è stato lo studio della mineralogia dei sedimenti fluviali ed eolici attuali generati dall’erosione della catena Himalayana, con lo scopo di definire con precisione le segnature composizionali dei diversi domini tettono-stratigrafici dell’orogene. All’approccio basato sulla identificazione e quantificazione delle associazioni di minerali pesanti (densità > 2.90 g/cm3), sono stati affiancate diverse tecniche analitiche complementari, che comprendono la petrografia e la geochimica del sedimento totale, lo studio di dettaglio al microscopio elettronico a scansione e allo spettroscopio Raman delle caratteristiche minerochimiche dei quattro principali gruppi di minerali pesanti che caratterizzano i sedimenti himalayani e orogenici in generale (anfiboli, epidoti, granati, e pirosseni), l’ analisi dei rapporti isotopici di samario e neodimio scolta in collaborazione con Peter Clift presso l’ Istituto Oceanigrafico Woods Hole, oltre all’ analisi geocronologica su zirconi detritici svolta in collaborazione con Pieter Vermeesch presso l’Università di Londra (UCL) affiancata anche da dati analoghi su rutilo, monazite, e titanite ottenuti con la collaborazione di Xiumian Hu e Ronghua Guo presso l’ Università di Nanjing. Le principali aree di studio hanno compreso il Deserto di Thal nel Pakistan centro-settentrionale e gli affluenti del Fiume Indo nel suo corso di montagna dal Ladakh fino al Punjab e gli affluenti principali del Fiume Yarlung (il nome tibetano del Brahmaputra) in Tibet meridionale. Sono stati studiati anche campioni di arenarie Cenozoiche provenienti sia dalle Alpi Occidentali che dal Tibet meridionale. I risultati presentati in questa tesi sono stati pubblicati a primo nome in un volume speciale della rivista internazionale Minerals, sono stati sottomessi nel mese di Settembre e sempre a primo nome a Sedimentary Geology, o sono in preparazione per una prossima sottomissione ad altra prestigiosa rivista internazionale.

(2020). Petrology and multimineral fingerprinting of modern sand derived from the Himalayan orogen. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2020).

Petrology and multimineral fingerprinting of modern sand derived from the Himalayan orogen

LIANG, WENDONG
2020

Abstract

Sediments and sedimentary rocks can be considered as geological archives that faithfully reflect their provenance information if the bias introduced by physical and chemical processes during transport and deposition can be properly recognized and corrected for. The sediment provenance analysis both in modern and ancient settings is crucial to trace the sediment sources, reconstruct paleoclimate and paleoenvironment, and interpret the evolution of the Earth’s surface. Modern sediments, unaffected by diagenesis and eroded, tansported and deposited under climatic conditions that are fully known, can provide valuable information on the interactions among the different controlling factors that govern source-to-sink systems. Rivers draining the Himalayan orogen provide the good opportunity to trace the source fingerprinting that is documented in modern fluvial and eolian sand and how these signatures reflect the erosion patterns of the modern and paleo-river systems. A multidisciplinary approach based on petrography, minerology, geochemistry and geochronology is emphasized in this research, in order to obtain a comprehensive provenance information. Our research area focused on the modern sands from two river system: Yarlung River and Indus River. In the Yarlung River system, the Nian River was chosen to investigate the petrographic, mineralogical and chronological signature of sediments from Tethys Himalaya, Greater Himalaya, Kangmar gneiss dome and Transhimalayan ophiolitic suture. Different tectonic domains are characterized by distinct heavy mineral assemblages, e.g., the first-cycle sillimanite and garnet in Greater Himalaya, and clinopyroxene, olivine and enstatite in the forearc ophiolites. Sand carried by the Nian River and its major tributaries, mainly reflects Tethys Himalayan characteristics, consistent with the geochronological results. Erosion rates were also evaluated and circumscribed in the middle Yarlung River catchment. The average erosion rate in the Nianchu catchment is estimated at 0.07-0.10 mm/a, twice as that in the middle Yarlung and Lhasa River catchments, which is principally ascribed to the high erodibility of Tethys Himalayan strata. In the Indus River system, minerochemical analysis of amphibole, garnet, epidote and pyroxene grains, and geochronological analysis of detrital zircons, associated with analysis on petrography, bulk-sediment geochemistry and isotopic geochemistry, in aolian sand from Thal Desert and fluvial sand in selected tributaries draining one specific tectonic domain in the upper Indus catchment, were carried out to discriminate compositional signatures, decipher the provenance information, and trace the erosional evolution of the western Himalaya syntaxis. The compositional fingerprints of Thal Desert sand are characterized by litho-feldspatho-quartzose to quartzo-feldspatho-lithic detrital modes and very rich amphibole-dominated heavy-mineral assemblages. The high heavy mineral concentration, less negative εNd, abundant zircon ages at 40-100 Ma, and specific mineral varietal fingerprints, consistently reflect that the Kohistan arc has acted as the main sediment source. Karakorum appears to contribute less while Himalaya shows higher influence on the Thal Desert sands than modern river sands from the Indus. As a Quaternary repository of wind-reworked Indus River sand at the entry point in the Himalayan foreland basin, Thal Desert sands document higher erosion rates than today in the glaciated areas formed largely by batholites granitoids of the Asian active margin. The close compositional and chronological connection between the Thal Desert and the ancient Indus Delta and Fan deposits, shed new light on the reconstructing of paleodrainage and the understanding of relationship between climatic and tectonic forcing that controlled the erosional evolution of the western Himalayan-Karakorum orogen.
GARZANTI, EDUARDO ALDO FRANCO
Fiumi dell'Himalaya; deserto di Thal; provenienza; metodi multiminerali; modello di erosione
Himalayan rivers; Thal Desert; provenance analysis; multimineral methods; modello di erosione
GEO/02 - GEOLOGIA STRATIGRAFICA E SEDIMENTOLOGICA
English
10-mar-2020
SCIENZE CHIMICHE, GEOLOGICHE E AMBIENTALI
32
2018/2019
open
(2020). Petrology and multimineral fingerprinting of modern sand derived from the Himalayan orogen. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2020).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/271022
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